This invention concerns taps for forming internal threads in parts. Production of internally threaded parts such as nuts involve a tap mounted on a tap driver. The nut blanks are fed onto a chamfered end of the tap, and driven down the tap length as the tap is rotated to cut the internal threads. The threaded nuts pass off the end of the tap and onto a shank, and out of the machine off an angled end of the tap driver. A surrounding drive tube having a slot wide enough to allow the nuts to pass through drives the angled end of the tap driver. Other styles of tappers exist.
Taps specially adapted to nut threading (often referred to as "nib" taps) are configured so that only one or perhaps two turns of the threads on the tap have a pitch diameter specified for the tap, with the remaining turns adjacent the shank end slightly undersized to enable free movement of the nuts onto the driver after the nut threads are fully formed.
The production of nuts intended for a particular application involves a determination of the exact pitch diameter of the tap necessary to produce a nut that will be properly fit to its mating component.
Nib taps are not typically stocked by manufacturers. Instead users must specially order such taps from a tap manufacturer, specifying one of a series of particular tap pitch diameter "size limits" related to a standard "basic" or "nominal" pitch diameter, which the user hopes will form an internal thread of a proper class of fit to the mating part. These series of size limits are specified in the United States by GH numbers, (for "ground high") such as GH1 to GH10 or greater. Undersize size limits are specified by GL numbers for "ground low". Each size limit number represents a successive step of 0.0005 inch in a range of size limits over the basic or nominal pitch diameter of the tap, for tap sizes up to 1 inch, with 0.001 steps for taps between 1 and 11/2 inches, 0.0015 inch steps for taps between 11/2 and 21/4 inches and 0.002 inch steps for taps between 21/4 and 4 inches in diameters.
In metric sizes, a similar system of "D" number size limits are used.
This situation leads to several problems for the tap user.
The need to special order nib taps causes delays due to the need for the taps to be manufactured to order, the manufacturing to order also increasing costs for the manufacturer and thus raising the price to the user.
As noted above, nib taps are made with a back taper in which the thread turns are of progressively smaller pitch diameter. The front end of the tap has the crests of the threaded turns ground off to allow entry into the bore of the nut blank and to distribute cutting pressure.
This typically results in only one or two tap thread turns having a pitch diameter at a specified GH size limit range.
Sometimes the tap user cannot obtain a properly sized thread by use of the taps of the GH number which he has ordered, because of the cutting characteristics of the particular material or due to another of the numerous factors affecting the tapping process. These special order taps can not be returned to the manufacturer and become useless for production on that job. The taps can then only be held in inventory by the user in the chance that they will be useable on a future job requiring the same basic pitch diameter size limits. The cost of taps is such that this alternative is unsatisfactory.
A more common problem is created by early wear of the one or two full pitch diameter turns establishing the pitch diameter of the cut thread. These turns are subject to early wear as they bear a disproportionate share of the cutting load, or forming load for swaging taps.
For nib taps, the full pitch diameter thread turn or turns are often located in the chamfer section, and hence these turns are performing substantial cutting of the thread form at the same time as they are finish sizing the pitch diameter. This situation often leads to early wear out of the pitch diameter sizing turns, so that correctly sized parts can no longer be produced from use of that tap.
As the tap sizing turns wear, the carefully determined size limit pitch diameter is lost, and the cut nut threads are no longer in an acceptable size limit range.
It is the object of the present invention to provide a nib tap configuration allowing a standard tap or a family of standard taps which can be stocked for off the shelf delivery and can be readily adapted by the user to a selected tap pitch diameter size limit.
If a tap is not usable on a given job, the tap may be adapted for potential use on any of a number of other jobs requiring taps in a range of size limits.
It is a further object of the invention to provide a method of making a tap of a selected size limit.
It is another object of the present invention to provide a tap configuration in which the problem of excessive wear of the tap threads at the pitch diameter size limit is substantially alleviated.